According to Jones (2002) functional foods are defined broadly as ‘foods that provide more than simple nutrition; they supply additional physiological benefit to the consumer.’ Prebiotics are a specific class of functional foods. According to Jones (2002) they may be defined as ‘an indigestible food ingredient that beneficially affects the host by selectively stimulating the growth or activity, or both, of one bacterium or a limited number of bacteria in the colon, thus improving the host's health’. Examples include: neosugars, inulin, soy hydrolysates, isomaltooligosaccharides, galactooligosaccharides, xylooligosaccharides, lactulose, raffinose, sorbitol, xylitol, palatinose and lactosucrose.
Prebiotic bacteria have a positive and beneficial effect in the gut of man. Examples are Lactobacilli spp and Bifidobacteria spp. More details may be found in recent reviews (Loo et al, 1999; Topping and Clifton, 2001; Tomasik and Tomasik, 2003). Both pre- and probiotic foods may be consumed together to promote the colonisation of probiotic bacteria in the gut.
Glucomannans are neutral polysaccharides produced by many plants where they serve as energy reserves and in some cases structural roles. The polysaccharides comprise, in most cases, predominantly mannose residues with glucose as the second sugar. The polysaccharides contain some acetylated residues and may contain some galactose side chains (Khanna, 2003). Sources of glucomannans are presented in Table 1.
TABLE 1Glucomannans from different sourcesDegree ofMannose:GlucosePolymerisationSourceRatio (MGR)(DP)Eastern white pine3.8:190(Pinus strobes)Higanbana4.0:1730(Lycoris radiata)Konjac1.6:1>6,000(Amorphophallus konjac)Lily2.7:1220(Lilium auratum)Orchid3.2:1600(Tubera salep)Ramie1.8:1nd(Boehmeria nivea)Redwood4.2:160(Sequois sempervirens)Suisen1.5:1nd(Narcissus tazetta)nd: not determinedAdapted from Khanna (2003)
From Table 1, it is evident that glucomannans can be extracted from a broad range of different botanical sources where there is variability in molecular weight and mannose to glucose ratio.
Konjac glucomannan is a polysaccharide extracted from the Amorphophallus konjac plant (or group of plants) which is, itself, a member of the Araceae genus. Konjac corms have been grown as food for centuries in Asia where they have provided a source of food with very interesting physical characteristics (Thomas, 1997; Khanna, 2003). The flour produced from konjac corns is used as a gelling and thickening agent and is a permitted food ingredient (Europe, E425). The principal polysaccharide component of konjac corn is a glucomannan that has exceptionally high swelling characteristics when hydrated. The flour has been used in gums throughout the world but use for this purpose was recently banned (e.g. Europe) because of the death of eighteen people as a consequence of choking. Other nutritional benefits of the flour include cholesterol lowering, bulking for weight reduction and to reduce the risk of constipation (Khanna, 2003). Apart from food uses, the flour may be used as a film forming material, pharmaceutical excipient, within body care products and as a chromatographic media (Khanna, 2003).
Commercial applications of konjac flour require different purity of the flour with the highest glucomannan content reflecting the highest cost. Purification is usually achieved with sieving procedures and alcohol washing rather than treatment with enzymes (amylases, proteases and lipases to remove non-glucomannan components) per se (Khanna, 2003).
Konjac glucomannans are high molecular weight polymers where the molecular weight typically exceeds 1×106 D (Khanna, 2003). The sugars are arranged in blocks of mannose and glucose residues that are β-(1-4) linked with typically 1.6:1 mannose to glucose residues within the polysaccharides. This linear structure is interspersed with branches on C3 of the sugar residues at approximately every tenth hexose unit with an esterified acetyl group at approximately every nineteenth residue (Khanna, 2003).
In the human body, some polysaccharides are defined as for example ‘starchy’, ‘digestible’ or ‘available’ whilst others are defined as for example ‘indigestible’, ‘non-digestible’, ‘dietary fibre’ or ‘non-starchy’. The starchy polysaccharides are digestible by mans' digestive enzymes in the small intestine if they are amorphous. If starchy polymers are crystalline they may be carried to the large intestine where they are fermented and are described as ‘resistant starch’. Non-starch polysaccharides cannot be digested in mans' small intestine and are always carried to the large intestine where they are fermented. Non-starch polysaccharides and resistant starch together form dietary fibre. This is an apparently important component of the diet as it promotes gut transit of food, provides bulk and satiety and provides a fermentation matrix in the colon. This fermentation releases short chain fatty acids which may be absorbed into the blood stream which is reported to have beneficial effects against gut cancer. Glucomannan polysaccharides would be fermented in the large intestine of man accordingly. For more details regarding the fermentation of polysaccharides in the gut readers are referred to a recent review by Topping and Clifton (2001).
In the present Application the term hydrolysate means material of lower molecular weight than the parent polysaccharide, and includes, but is not limited exclusively to, oligosaccharides and sugars.
The production of hydrolysates from polysaccharides can be achieved by, for example, acid or enzymatic hydrolysis. With respect to mannans (including glucomannans and especially konjac glucomannans) this may be achieved under appropriate conditions too. Acid hydrolysis tends to be random whilst enzymatic hydrolysis is more focused towards specific bonds. Konjac (and non-konjac) glucomannan may be hydrolysed by acids, mannanases and cellulases (Kato and Matsuda, 1969; Kato et al, 1970; Shimahara, 1975; Chiu et al, 1991; Ohya et al., 1994; Behr, 1998; Chiu et al, 1998; Edashige and Ishii, 1998; Kurakake and Komaki, 2001; Cescutti et al, 2002; Chiu et al, 2002; Qi et al, 2003). Hence, the use of cellulases and other enzymes for the purpose of konjac glucomannan hydrolysis has been well established before 1980. It is, perhaps, unusual that the patent described by Chiu et al (1991, 2002) was granted. This concerns the use of cellulases to hydrolyse konjac glucomannans and their use as potential non-digestible bulking agents in foods.
Water soluble konjac (dialysis) extracts have been investigated and published by Shimizu Manzo Shoten KK company (1974) and Sugiyama and Shimahara (1976). Gel systems based on konjac and starch combination have been discussed by Tye et al (1990, 1994). Chiu et al (1991, 1998, 2002) have discussed the use of cellulases to convert konjac glucomannans to oligosacccharides and their use as sugar replacements/bulking agents. King et al (1994) and Wheatley et al (1996) have used konjac glucomannan as a sustained release excipient. Healthcare drinks/compositions containing konjac with or without the addition of other health promoting components have been described (Zhou, 1995; Vuksan, 2001; Sun, 2003).